1. Field of the Invention
The present invention relates to casting metal objects in molds that have riser cavities and riser toppings in the riser cavities, and to the problems associated with the use of riser toppings.
2. Description of the Art
In some metal molding processes, such as those used in casting steel railroad wheels, a solid graphite mold is used, with both cope and drag portions. Such molds are designed to be reused after a cast wheel has been removed from the mold. The cope mold portion, that is the top portion, typically has riser cavities. Such riser cavities typically have generally vertically-disposed cylindrical walls, and are open at both the tops and bottoms of the cavities. Prior to casting, a sand coating is typically baked onto parts of the walls of the riser cavities. During casting of the wheel, a reservoir of molten metal forms in the riser cavity attached to the wheel casting to compensate for internal contraction of the casting during solidification. After the metal is poured, a riser topping material is typically placed in the open top of the riser cavity on top of the molten metal in the riser. Riser toppings provide an insulating effect and reduce heat loss from convection and radiation. Compounds that serve as such insulators include powdered graphite, coke breeze, charcoal, rice or oat hulls and various combinations of refractory powders. After the cast wheel is removed from the mold, there is a waste metal riser left in the riser opening, as well as waste riser topping material. Before the cope portion of the mold is used again, the waste riser and waste riser topping material must be removed from the riser cavity, and the baked sand coating must be removed.
In one prior art system, the waste metal riser and waste riser topping material are knocked out of the riser opening by a plunger assembly. In that system, there are brushes attached to solid plungers that are driven through the riser cavities of the cope portion of the mold. The plungers push the waste risers out through the bottoms of the riser cavities, the riser topping materials drop out through the bottoms of the riser cavities, and the brushes clean the cylindrical side walls of the riser cavities, brushing off the baked sand coating on the walls of the riser cavities. In this system, the waste risers and some of the riser topping materials drop through a chute in the mill floor below the apparatus. There are hoppers in the basement under the mill floor and riser knock out station. Deflector bars beneath the mill floor deflect the waste risers to one hopper and allow sand and riser topping to drop to a second hopper. However, quantities of riser topping material also frequently fall to the mill floor. In another prior system, the surface of the cope mold portion is cleaned with high pressure air streams which blow any riser topping materials off of the cope mold portion surface. The waste metal risers may be knocked out in a separate operation and the waster metal risers and riser topping materials may then be gathered from separate hoppers below the mill floor, and the waste metal risers may be recycled. However, quantities of riser topping material also frequently fall to the mill floor. With these prior systems, the riser topping materials, such as the rice hulls, create potential maintenance and safety problems. The hulls or other riser topping material can get into the plant machinery, such as the conveyor, and dust from the riser toppings can get into the workers' eyes, for example. In addition, the waste riser topping material must be gathered up from the mill floor in a labor-intensive operation.
In typical production, the cope mold sections are recycled for reuse after the risers have been pushed out of the riser cavities.